A spark-ignition internal combustion engine is powered by the ignition of combustion mixture which develops pressure to pistons driving the crank shaft. The power delivered to the piston will be maximum if it is driven at the time when the piston is positioned at the top dead center. Ignition of fuel in the combustion chamber occurs when a spark is generated across the spark plug at a specific time controlled by the ignition system. Since the pressure of the combustion need some times to develop after the ignition. To maximize the power, the ignition should occur before the piston reaching the top dead center.
The ignition control system is responsible for the generation of ignition; the determination of the exact timing for the ignition and the management required for the related devices such that they can work efficiently.
There are two common ignition generation methods.
The first type of the ignition generation system is the contact-breaker system in which timing pulses are generated by a contact-breaker mechanically coupled to the crank shaft of the engine, normally the contact is closed to allow current to flow through the ignition coil. Reaching the right time for ignition, the contact is broken, magnetic field in the coil collapses suddenly, a high voltage needed for the ignition is induced in the secondary winding of the ignition coil. Some systems employ beaker-less electronic devices to substitute for the contacts which may wear out, these electronic devices include magnetic induction; hall effect; optical pickup devices which practically have no wear out. The problem of this type of ignition system being extra measures has to be taken to handle the dwell angle since current has to remain flowing through the ignition coil in order to maintain the magnetic field. Dwell angle control is employed to determine the optimum time when to switch on the current before the next ignition otherwise the power dissipation of the ignition coil is accountable.
The second type of the ignition generation system is the capacitor discharge system in which the spark for ignition are generated by the rapid discharge of a charged capacitor through the ignition coil. The advantage of such system being, no power dissipation after the capacitor is fully charged because the charging current ceases, and the energy stored in a capacitor is the energy for sparking. It remains relatively constant until it reaches the high engine speed, at which the charging time for the capacitor is not enough.
Such ignition systems described above offer only the high energy sparks for ignition, the time at which such sparks should occur has to be precisely controlled in order to achieve high engine efficiency. Assuming that the time required for the pressure of the ignited mixture to develop is constant, the higher the engine speed, the position of the piston should be further away before reaching the top dead center. This position is normally interpreted as angular position and is commonly called the ignition advance angle or simply ignition advance.
Actually, many factors affect the time required for the combustion to deliver its maximum power, and the ignition advance should change according to the rotational speed of the engine, normally the ignition advance increases with a steeper slope at the lower speed and gradually become less steep at the higher speed.
Motor cars are spacious enough to handle such advance mechanically, they have the centrifugal ignition advance control to compensate for speed changes and the vacuum ignition advance control to compensate for air fuel ratio. However, those devices are either not accuracy enough or they cannot respond to rapid changes at which states the fuel wastage are obvious.
Most of the motorcycle ignition advance are fixed values, for example 18 degrees before top dead center BTDC at idle speed and remains unchanged though out the operating range. This simple fact indicates that most of the motorcycle are not working in the optimum performance.
Ignition advance control is to compensate the time required for the ignition of fuel to reach its maximum pressure; dwell angle is to control the time before supplying current to the ignition coil so that the power dissipation in the coil is minimized. Both are functions of time. In the traditional engines, time T can only be represented by an angle D (degree) while the engine is running at the rotational speed R.P.M.(revolution per minute) EQU D=[T.times.R.P.M..times.360/60]
for example if the time required for the combustion pressure to develop is 4 milliseconds, then at 1000 R.P.M., by advancing the ignition by 4 milliseconds means the ignition should occur while the crank shaft is positioned at 24 degrees before reaching the top dead center. Ignition advance angle and dwell angle should be defined together with the R.P.M. the engine is running at. These values are commonly represented by curves with ignition advance angle and dwell angle vs. The engine rotational speed.
If there exist a set of optimum ignition advance angles for various operation conditions represented as curves, then if the ignition system is triggered at the angle as indicated in the curves, the ignition of the engine are said to be optimized. Practically curves for various operation conditions such as temperature, humidity, octane value, etc. can be obtained from tables stored in read only memory device, or they can be determined by computing devices which connect directly to sensors tracing the related physical quantities. An angular position reference precise enough to indicate the angle for the ignition is needed.
For every ignition coil, there exist a best charging time for the magnet field at different supply voltage. If these values are transformed to angle C at different engine rotational speeds, thus by storing P=90.degree.-C (4 cylinders) or P=60.degree.-C (6 cylinders) in the read only memory device, by converting the analog values of the supply voltage to digital values, then by looking up tables that contain the values for the angle P for different supply voltages and engine rotational speeds, Dwell angle can be controlled if a precise enough angular position reference is available.
Improving the efficiency of engines is the modern trend, it helps to preserve energy and to protect the environment. Other than changing the compression ratio; adding valves etc. mechanically, modern engine design uses two major approaches to achieve engine improvement.
Firstly, high precision ignition control is employed. The modern ignition control system requires that the accuracy of such reference to within 1 degree. The control unit can that compute or lookup tables to determine the optimum ignition angle.
Secondly, high precision fuel control is employed. The best fuel to air ratio; injection, etc., are controlled by the computer according to the operating conditions.
Both measures required entirely new devices. Commonly, the ignition timing signal is not coming from the distributor but a new crank shaft position detector; the fuel is not supplying from the carburetor but the fuel injection devices. These new measures are not for traditional engine which can only be working inefficiently, polluting the environment until they extinct.
An object of the invention is therefore to provide an angular position prediction device which derives a pulse sequence synchronous with the rotation of the engine from the original ignition timing signal such that optimum ignition for any engine is possible.